1. Field of the Invention
The field of the present invention is electronics. More particularly, the present invention relates to direct conversion receivers in wireless communication devices.
2. Description of Related Art
Most present wireless communication devices use transceivers (transmitters and receivers) that have an intermediate frequency (IF) stage between the baseband and the radio frequency (RF) stages. Transceivers with an IF stage are called superheterodyne transceivers. The compelling commercial drive for cheaper, more reliable, longer lasting and smaller wireless communication devices is causing many in the industry to attempt to eliminate the IF stage. This would produce a saving in number of components, cost and size.
Transceivers without IF stages are called direct down conversion transceivers, since the RF signal is converted directly to a baseband signal from the RF signal. They are also known as zero IF transceivers.
Since a superheterodyne receiver does have an intermediate frequency stage, a superheterodyne receiver will generally have more components compared to a direct downconversion receiver. Currently, most wireless handsets are made with superheterodyne transceivers (transmitter and receiver) since the use and manufacture of superheterodyne receivers is well understood. Although most handsets use a superheterodyne design, such use is in tension with the ever-present concern of reducing the size of handsets and lowering their cost of manufacture because of the hardware needed for the intermediate frequency stage.
The use of direct-conversion technology in wireless handsets would obviate the need for an intermediate frequency stage, thereby reducing manufacturing cost and size limitations. However, a number of problems have prevented the widespread use of direct conversion technology in wireless handsets. For example, consider FIG. 1 showing a prior art direct downconversion receiver 5. The receiver 5 includes an antenna 10 to receive a transmitted radio-frequency (RF) signal 11. RF signal 11 couples through a duplexer 12 to a low noise amplifier (LNA) 14. The amplified RF signal then couples to a mixer 15 through RF port 22. The amplified RF signal is typically a bandpass signal, gbp(t), that may be represented asgbp(t)=gc(t)cos w0t+gs(t)sin w0twhere gc(t) and gs(t) are the in-phase (I) and quadrature (Q) components of the baseband signal, respectively. Thus, to convert this bandpass signal to its baseband components, a voltage-controlled oscillator (VCO) 16 produces a sinusoid at the RF frequency w.sub. 0 to couple into the mixer 15 through LO port 24. A baseband low pass filter 18 recovers the baseband components, which are then processed by an A/D and digital-signal-processor (DSP) baseband processor 20.
Note that the mixer receives sinusoids at the same frequency, w0, at both its ports 22 and 24. Thus, unlike a mixer in the IF stage of a superheterodyne receiver, serious coupling side effects can occur in the mixer 15. These effects include non-linear effects of the mixer producing unwanted harmonics of the baseband signal. In addition, a DC offset may be present in the demodulated baseband signal due to leakage of the VCO's sinusoid output into RF port 22 of mixer 15. This type of leakage is particularly problematic because the VCO output is typically many decibels higher in power than the output of the low noise amplifier 15. Moreover, this type of leakage is exacerbated at the higher frequencies, such as the PCS band, used in wireless handsets.
Given an LO leakage, a sinusoid output at frequency w1 from the VCO 16 entering LO port 24 will also couple into RF port 22. The same signal is thus present at both RF port 22 and LO port 24 and will be squared by mixer 15. Regardless of the phase of the input sinusoid, its squaring produces a sinusoid of double the input frequency and a DC offset term. Thus, LO leakage necessarily produces a DC offset. A number of techniques have been developed to address the problem of LO leakage and the resulting distortion and DC offset in the demodulated baseband signal. For example, U.S. Pat. Nos. 4,811,425 and 5,001,773 disclose schemes to directly cancel the LO component entering RF port 22 of mixer 15. Because the LO component entering RF port 22 is necessarily of smaller amplitude than the LO signal produced by VCO 16, a cancelling signal must be appropriately scaled and phase-shifted to cancel the leaking LO component. These direct cancellation schemes suffer from the expensive hardware necessary and poor efficiency at cancelling the leakage component.
Accordingly, there is a need in the art for improved apparatus and techniques for reducing local oscillator leakage effects in direct downconversion receivers.